At the Republican retreat for members of Congress in Philadelphia, President Trump's tweets, speeches and executive orders derailed the GOP's plan to agree upon a replacement for Obamacare and set other policy initiatives. (Jayne Orenstein/The Washington Post)

If President Trump is waiting for the Republican Congress to join him in his quixotic quest to launch the first investigation in American history that will uncover systematic voter fraud  — well, he may be waiting a while.

ABC reported Wednesday that Trump would like Congress’s help as he launches a “major” investigation to find the 3 million to 5 million votes he claims were cast illegally. For a variety of political and logistical reasons — but mostly political — Congress will almost definitely not have Trump’s back.

“We haven’t been discussing that,” House Speaker Paul D. Ryan (R-Wis.) said in an interview Wednesday with MSNBC’s Greta Van Susteren when asked whether Congress would join in.

“The president has 100,000 people at the Department of Justice, and if he wants to have an investigation, have at it,” Rep. Jason Chaffetz (R-Utah), chairman of one of the top House investigative committees, told CNN.

The Washington Post's Paul Kane speaks to Libby Casey about President-elect Donald Trump's unfounded claim that millions of people voted illegally for Hillary Clinton. (The Washington Post)

UPDATE: At least one Republican in Congress is bucking this trend. Rep. Mo Brooks (R-Ala.) offered to help Trump, according to a recording from a closed-door Republican retreat, complaining of alleged voter fraud in his own district. Brooks has declared there is a "war on whites," especially with regard to accusations of racist comments dogging his fellow Alabamian, Sen. Jeff Sessions, who is Trump's pick to be attorney general.

Brooks aside, there are plenty of good reasons Republicans in Congress aren’t jumping at the chance to help their president uncover voter fraud. Here are seven of them:

1. It would give Democrats a very public platform to make the case that numerous political scientists, academics and politicians have already made: There is no evidence of widespread voter fraud.

But since we’re discussing voter fraud, Democrats may say, let’s talk about all the voter-ID laws Republicans have passed, which research shows disproportionately affect the poor, people of color and the elderly and young people.

“If Democrats prepare for them right, televised hearings could be both an opportunity to deflate the voter-fraud myth and an opportunity to spotlight various tactics of disfranchisement,” said Cornell Law professor Josh Chafetz.

2. It risks undermining the nation’s 31 Republican secretaries of state: Elections — even presidential ones — are administered at the local and state level. A secretary of state’s job is to ensure a smooth and fair election.

Trump is alleging that the number of illegal votes may have been larger than the population of 38 states, points out The Fix’s Aaron Blake. (The fraudulent-vote figure that Trump has been bandying about is also roughly equivalent to the margin by which he lost the popular vote.) The massive number of fraudulent votes Trump is alleging means he’s calling into question the way many Republican officials oversaw their elections.

Meanwhile, those Republican secretaries of state are pushing back on the notion of voter fraud in their states:

3. It could even risk undermining Trump’s presidency. Not officially, of course — just in the public imagination (and, very likely, Trump’s). In Trump’s interview with ABC News on Wednesday, he brought up twice — unprompted — the notion that not a single vote was cast illegally for him. But that logic doesn’t hold up: If fraud is so common, as Trump claims, is it a realistic assumption that not a single fraudulent vote went to Trump? Under that logic, you could even argue the opposite is possible: That widespread fraud helped Trump, who won Wisconsin, Michigan and Pennsylvania by less than a percentage point each.

4. You never know what an investigation will find. “There always is some downside to letting a bunch of lawyers loose,” said Linda Fowler, professor of government at Dartmouth College. Or at the very least, surprises. She pointed out that a House special investigation into the 2012 terrorist attacks in Benghazi, Libya, uncovered then-secretary of state Hillary Clinton’s use of a private email server. And the Whitewater investigation into the real-estate deals of the Clintons set in motion the chain of events that uncovered President Bill Clinton’s affair.

5. It would take away from other things Congress wants to do. A high-profile voter fraud investigation is costly, timely and, frankly, could be a distraction for actual legislation GOP leaders want to pass now that they finally control Washington — like tax reform and Obamacare repeal; Trump’s tweets about voter fraud (and a whole bunch else) are proving to be distraction enough.

6. Congress doesn’t really investigate local issues. It’s hard to see what committee would have jurisdiction over 51 elections, Fowler said. Usually when Congress investigates something, it’s the executive branch in the hot seat.

7. There is just no evidence of voter fraud. Why launch an investigation into something that nearly everyone in U.S. politics — save one notable exception — doesn’t believe warrants an investigation?