U.S. Senate
Despite vowing not to return to elected office, Alexander was nevertheless persuaded by the White House to run for the open seat of retiring Senator Fred Thompson in 2002. Seen as a moderate Republican by Tennessee standards, Alexander’s candidacy was vigorously opposed by conservatives, who instead supported US Representative and House manager during the 1998 impeachment of Bill Clinton, Ed Bryant.
Alexander was better-funded and armed with more prominent endorsements, however, and edged Bryant in the primary, 295,052 votes to 233,678. Democrats had high hopes of retaking the seat that they lost in 1994 with their candidate, US Representative Bob Clement, a member of a prominent political family. However, Clement's campaign never really caught on, and Alexander defeated him in the general election with 54 percent of the vote. With his election to the US Senate, he became the first Tennessean to be popularly elected both governor and senator. At the age of 62, Alexander also became the oldest elected freshman US senator from Tennessee since Democrat Lawrence D. Tyson in 1924, which he held until 2018 when Marsha Blackburn surpassed him at the age of 66.
In April 2007, Alexander announced he would run for re-election to the Senate in 2008.
Alexander was favored throughout the entire campaign, due to his long history in Tennessee politics and a disorganized Democratic opposition. His rivals were former state Democratic Party Chairman Bob Tuke, who won a heated primary, and Libertarian candidate Daniel T. Lewis.
Alexander won reelection, taking 65 percent of the vote to Tuke's 32 percent. Alexander also carried all but one of Tennessee's 95 counties; he lost only in Haywood County in western Tennessee, which was secured by Tuke. He won the normally Democratic strongholds of Davidson and Shelby counties—home to Nashville and Memphis, respectively. Alexander also benefited from riding the coattails of John McCain, who won the state with a solid majority.
In December 2012, Alexander announced he would be seeking re-election to a third Senate term in 2014. Alexander's campaign had a war chest of $3.1 million in cash going into his 2014 re-election bid.
In an August 2013 letter to Alexander signed by over 20 Tennessee Tea Party groups, the groups called on Alexander to retire from the Senate in 2014, or face a primary challenge. The letter stated: "During your tenure in the Senate we have no doubt that you voted in a way which you felt was appropriate. Unfortunately, our great nation can no longer afford compromise and bipartisanship, two traits for which you have become famous. America faces serious challenges and needs policymakers who will defend conservative values, not work with those who are actively undermining those values."
Although Alexander was initially thought to be vulnerable to a primary challenge from the right, he worked to avoid this and ultimately did not face a high-profile challenger. He declared his intention to run early, quickly won the endorsement of Governor Bill Haslam, every living former Tennessee Republican Party chair person, and the state's entire Republican congressional delegation, except for then scandal-hit Scott DesJarlais. He also raised a large amount of money and worked to avoid the mistakes of ousted Senators Bob Bennett and Richard Lugar by trying to stay in touch with his constituents, especially in East Tennessee. Moreover, out-of-state conservative organizations such as the Senate Conservatives Fund made little effort to defeat Alexander.
Alexander won the Republican primary, defeating State Representative and Tea Party challenger Joe Carr. However, Alexander recorded the lowest winning percentage (49.7%) and lowest margin of victory (9.2 points) ever in a primary for a Republican U.S. senator from Tennessee. Carr won a larger percentage of the vote (40.5%) than the previous 11 challengers to sitting Republican U.S. senators in Tennessee history combined (40.3%). Alexander won the general election with 62% of the vote.
In 2006, a newly discovered species of springtail found in Great Smoky Mountains National Park was named Cosberella lamaralexanderi in Alexander's honor, because of his support of scientific research funding in the park and because the springtails' patterning is reminiscent of the plaid shirts Alexander typically wears while campaigning.
On October 6, 2018, Alexander was one of 50 senators (49 Republicans, 1 Democrat) who voted to confirm Brett Kavanaugh to the Supreme Court.
In late 2006, Alexander announced that he had secured the requisite number of votes to become the Republican Party's Minority Whip in the Senate during the 110th Congress. Even though he was seen as the preferred choice of Minority Leader Mitch McConnell and the Bush administration, he lost the election to former Senate Majority Leader Trent Lott by one vote (25–24).
Alexander would get a second shot at entering his party's leadership a year later when Lott announced his intent to resign from the Senate by the end of 2007. Sen. Jon Kyl of Arizona, then Chairman of the Senate Republican Conference, ran for Whip and was elected without opposition. With the Conference Chair vacant, Alexander announced that he would seek the position. He would go on to defeat Sen. Richard Burr of North Carolina by a margin of 31–16.
Alexander stepped down as Conference Chairman in January 2012, citing his desire to foster consensus. He said, "I want to do more to make the Senate a more effective institution so that it can deal better with serious issues." He added, "For these same reasons, I do not plan to seek a leadership position in the next Congress", ending speculation that he would run for the position of Republican Whip after Jon Kyl retired in 2013.
On December 17, 2018, Alexander announced that he would not seek another term in 2020. In an interview with Politico, he stated that he had made the decision as early as August 2018.
For his tenure as the chairman of the Senate Health, Education, Labor, and Pensions Committee in the 116th Congress, Alexander earned an "F" grade from the non-partisan Lugar Center's Congressional Oversight Hearing Index.
As co-chairman of the Joint Congressional Inaugural Committee, Alexander was one of the speakers at the Second inauguration of Barack Obama on January 21, 2013, alongside the committee's chair, Senator Charles Schumer.
Committee on Appropriations
Subcommittee on Commerce, Justice, Science, and Related Agencies
Subcommittee on Defense
Subcommittee on Energy and Water Development (chairman)
Subcommittee on Interior, Environment, and Related Agencies
Subcommittee on Labor, Health and Human Services, Education, and Related Agencies
Subcommittee on Transportation, Housing and Urban Development, and Related Agencies
Committee on Energy and Natural Resources
Subcommittee on Energy
Subcommittee on National Parks
Subcommittee on Public Lands, Forests and Mining
Committee on Health, Education, Labor, and Pensions (chairman)
Subcommittee on Children and Families (Ex Officio)
Subcommittee on Employment and Workplace Safety (Ex Officio)
Subcommittee on Primary Health and Aging (Ex Officio)
Committee on Rules and Administration
International Conservation Caucus
Sportsmen's Caucus
Tennessee Valley Authority Caucus (Co-chair)
The following is an incomplete list of legislation that Alexander introduced in the Senate.
PREEMIE Reauthorization Act (S. 252; 113th Congress) – a bill that would reauthorize research programs on preterm births that are run by the Centers for Disease Control and Prevention. It would also authorize grants and demonstration programs to be run by the Health Resources and Services Administration that will try to decrease preterm births. It passed the Senate on September 25, 2013.
Exchange Information Disclosure Act – a bill that would require the government to report on the number of visitors and enrollees on the federal government's healthcare exchanges, as well as what level of insurance coverage people buy on the exchanges. The bill would apply only to the federally run healthcare exchanges, which cover 36 states, not the state-run exchanges, according to Ripon Advance. On January 16, 2014, the U.S. House passed the bill. Its companion bill Exchange Information Disclosure Act (H.R. 3362; 113th Congress) was introduced in the House by Rep. Lee Terry (R, NE-2).