With U.S. President Donald Trump's impeachment trial set to begin in the Senate, some Republican allies continue to promote a discredited theory that accuses Ukraine of interfering in the 2016 U.S. election to keep him from winning.

The notion, which is not supported by U.S. intelligence agencies, has nonetheless been embraced by a president reluctant to acknowledge the reality of Russian election interference, and anxious to show he had reason to be suspicious of Ukraine as the U.S. withheld crucial military aid last year.

The effect: blurring the facts of the impeachment case for many Americans before it even reaches trial.

"The ultimate victim is democracy, is the stability of our nation," said Nina Jankowicz, a disinformation expert at the nonpartisan Wilson Center, a Washington, D.C.-based think-tank .

The president's demand that Ukraine look into its own purported interference and investigate a political rival, former Vice-President Joe Biden, while the U.S. withheld the aid is at the heart of the congressional investigation that produced Trump's impeachment in the House on charges of abuse of power and obstruction of Congress.

An Associated Press review shows the idea of Ukrainian interference took root during Trump's presidential campaign, was spread online and then amplified by Russian President Vladimir Putin before some of America's elected officials made it their truth.

As U.S. authorities collected evidence in 2016 that Russia had hacked Democratic National Committee servers, former Trump campaign chairman Paul Manafort suggested Ukraine, not Russia, had likely committed the attack, his deputy Rick Gates later told the FBI.

That September, Trump confidant Roger Stone tweeted, "The only interference in the U.S. election is from Hillary's friends in Ukraine," referring to Trump opponent, Hillary Clinton.

But Trump's FBI director rejected those allegations. U.S. intelligence agencies blame Russia for interfering on Trump's behalf, and special counsel Robert Mueller has charged 25 Russians with hacking Democratic email accounts and waging a covert social media campaign to sway public opinion.

Yet, against all evidence, the theory's shape-shifting nature over the years has compounded its staying power.

Stone's 2016 tweet, for instance, referenced a nebulous type of "interference," centred around Ukraine officials reportedly favouring Clinton over Trump.

The tweet highlighted a Financial Times article that described efforts by ex-Ukrainian parliament member Serhiy Leshchenko, who opposed Trump's bid, to expose off-the-books payments by Ukraine's pro-Russia political party, including to Manafort.

Leshchenko maintains his efforts don't amount to interference. Still, some Republican lawmakers, including a few contacted by AP, have cited the article to support the Ukraine interference argument.

"I think both Russia and Ukraine meddled in the 2016 election," Republican Sen. John Kennedy of Louisiana said last month on "Meet the Press."

Internet suspicions casting doubt on Russia's hack of the DNC and Clinton campaigns intensified as Trump prepared to take office.

"So how and why are they so sure about hacking if they never even requested an examination of the computer servers? What's going on?" Trump tweeted on Jan. 5, 2017, the day after a BuzzFeed News article revealed the FBI did not physically examine the Democrats' servers to determine Russia infiltrated the system

In February 2017, Putin publicly claimed Ukraine's entire government had supported Clinton and now needed to "improve relations" with the new Trump administration.

By that April, Trump himself promoted the theory, falsely suggesting in an AP interview that CrowdStrike, the cybersecurity firm that traced the hack to Russia, had strong ties to Ukraine.

"I heard it's owned by a very rich Ukrainian, that's what I heard," Trump said. "Why didn't they allow the FBI in to investigate the server?"

In fact, CrowdStrike is a California company founded by two U.S. citizens -- George Kurtz and Dmitri Alperovitch, who was born in Russia and lives in America.

And the FBI didn't need to physically take the DNC servers to confirm CrowdStrike's findings. CrowdStrike gave the FBI digital images that captured everything from emails, browsing history and files of the DNC system, the company says.

But Trump took his suspicions about the servers directly to newly elected Ukraine president Volodymyr Zelenskiy in a now infamous July 25 phone call that spurred the articles of impeachment against Trump.

"I would like you to find out what happened with this whole situation with Ukraine, they say CrowdStrike ... I guess you have one of your wealthy people..." Trump asked. "The server, they say Ukraine has it."

CrowdStrike released a blog post rebuffing Trump's claims. The president's own advisers rebutted the theory to no avail, former White House aide Fiona Hill told impeachment investigators.

Trump keeps the claim alive. He insisted to Fox News viewers in November that he only withheld aid from Ukraine to investigate corruption there, hinting once again that's where the DNC's servers are hidden.

"You know, the FBI has never gotten that server," Trump said. "That's a big part of this whole thing. Why did they give it to a Ukrainian company?"

Parts of the Ukraine theory have been echoed by the president's Republican allies -- some of whom concede Russia interfered but posit Ukraine did too.

"Russia's campaign to interfere in our election was real and systematic. It is also true that Ukrainian officials did not want...then-candidate Trump to win. The two are not mutually exclusive," the office of Republican Sen. Ted Cruz said in a statement. Cruz himself has said there's "considerable evidence" of Ukraine interference.

As his Senate trial nears, Trump has pressed GOP senators to rally behind him -- asking personal attorney Rudy Giuliani to brief them on his trip to Europe, where he searched for witnesses and documents.

Hill, a Russia expert, told Congress in November that political leaders who spread such falsehoods about Ukraine polarize the U.S., making it a target for misinformation campaigns by foreign powers like Russia.

She warned: "These fictions are harmful even if they are deployed for purely domestic political purposes."

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Associated Press writers Mary Clare Jalonick in Washington and Yuras Karmanau in Kyiv contributed to this report.