Tennessee lurched toward its most competitive Senate campaign in more than a decade on Thursday when voters chose a moderate Democratic former governor and a hard-charging Republican congresswoman as the nominees to fill the seat of a retiring lawmaker who has repeatedly clashed with the Trump administration.
Democrats nominated Phil Bredesen, who served two terms as governor, while Republicans chose Marsha Blackburn, a stalwart conservative who represents Middle Tennessee and has already won President Trump’s endorsement.
Neither faced competitive primaries, and The Associated Press projected both as winners less than 15 minutes after the polls closed across the state.
Thursday’s Republican primary for governor, featuring four major candidates jockeying for the nomination, was expected to be far more competitive.
President Trump is expected to be a factor in both races, but especially the Senate contest, in which he has already campaigned for Ms. Blackburn. She and Mr. Bredesen are hoping to assume the seat of Senator Bob Corker, the retiring chairman of the Senate Foreign Relations Committee and one of Mr. Trump’s most persistent Republican critics on Capitol Hill.
Installing Ms. Blackburn in the Senate would give the White House a new ally in a bitterly divided chamber. The campaign is expected to be among the most closely fought in the country: Mr. Bredesen is well known and well liked, but Tennessee has a conservative bent. Republican presidential nominees have carried it since 2000, and their party controls the Statehouse in Nashville.
For Republicans, the six-way primary in the governor’s race renewed tensions about the direction of the party in a state that has long embraced moderate conservatives in the mold of Howard Baker, the longtime senator, and more recently Mr. Corker and Mr. Haslam.
“There’s been this battle for a long time between the pragmatists and the purists,” said John G. Geer, a political scientist and the dean of the College of Arts and Science at Vanderbilt University in Nashville.
There were certainly echoes of that clash in the Republican race for governor, a $46 million contest where candidates galloped rightward and immigration became a central issue.
Diane Black, first regarded as a front-runner, struggled late in the campaign as the race became more of a free-for-all than a coronation of any single candidate. Ultimately, some Republicans supported Ms. Black’s rivals largely on the belief that they would fare better in November.
And the White House, which has proved itself a pivotal arbiter in some Republican primaries, remained largely quiet after some leading Republicans in Tennessee and Washington, uneasy with the hard-right conservatism of Ms. Black, privately signaled their preference that Mr. Trump stay out of the race.
He did so, but Vice President Mike Pence endorsed Ms. Black, a former House colleague, in a post on Twitter last week, praising her as a “strong supporter” of the White House’s agenda.
Mr. Pence’s carefully calibrated tweet on her behalf was seen by White House aides as a way of fulfilling his personal commitment to Ms. Black without involving the president in a race that the West Wing saw little upside to engaging in.
The bifurcation was somewhat odd — a vice president taking sides in a contested party primary but not the president — yet it came as something of a relief to Republican officials, some of them said. They had feared that Mr. Trump would weigh in, or that Mr. Pence would offer a full-throated appeal for Ms. Black while visiting Tennessee late last month, and were satisfied about the more cautious intervention.
The restraint, though, was not for the congresswoman’s lack of trying: She personally lobbied Mr. Trump for his support and also deployed some of her House colleagues to make the case to the president, according to a White House official.
And her advertisements made no secret of her loyalty to the president: One recent commercial depicted three separate meetings between Mr. Trump and Ms. Black.