Tucker also said he had hoped Trump would "stay out of the primary," but that his involvement "allowed Tillis to wrap himself in Trump's endorsement" and made it more difficult to challenge Tillis on his record. He also blamed the impeachment inquiry, arguing that the process was a "boon" to incumbent senators and allowed Tillis to appeal to the party base by defending the president. Tillis has been sharply critical of the impeachment inquiry.

"Impeachment has dramatically sucked the oxygen out of our campaign discussion and greatly curtailed our fundraising ability," Tucker wrote in the letter.

"As a result of this combination of factors, I have very reluctantly decided not to file as a candidate," Tucker added in the letter. "While I have been genuinely honored by your support to date and feel a real sense of obligation to you as donors, I cannot in good conscience solicit additional contributions when it is so apparent that sufficient funding is not available to see the campaign through successfully."

Tillis' campaign announced in September that it was booking $2.2 million on the airwaves, running TV ads touting his support from Trump and pushing back against Tucker's attacks.

Luke Blanchat, Tillis' campaign manager, said in a statement that with the primary challenge behind the campaign, "it's now time to turn out attention to building the diverse coalition necessary to defeat whichever radical, liberal nominee the Democrats put forward in March."

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Tucker loaned his campaign more than $1.1 million as of the end of September, and his campaign had spent more than $1 million on TV advertising boosting himself and attacking Tillis. But he did little to gain ground in the polls against the incumbent senator. A Fox News poll last month showed Tillis with a large lead over Tucker among GOP voters, 54 percent to 11 percent.

Tillis still faces farmer Sandy Smith, who had just $66,000 in cash on hand as of Sept. 30, in the GOP primary. The filing deadline for other candidates to join the race Dec. 20.

Tillis is still likely to face a competitive general election matchup. The Democratic Senatorial Campaign Committee endorsed Cal Cunningham, an Army veteran and former state senator who filed Monday to appear on the ballot. Cunningham still faces a competitive primary race against state Sen. Erica Smith.

Alex Isenstadt contributed to this report.