The unpredictable investigation into Russian meddling in last year's presidential election, now reportedly reaching into President Trump's White House, could land like a political nuclear bomb on the Republican Party in 2018.

That's what worries Republicans in Congress now. They had initially expressed relief about the Justice Department's appointment of former FBI Director Robert Mueller as special counsel to lead the inquiry, believing that it might free them, and voters, to focus on their ambitious legislative agenda.

"There are all kinds of unintended consequences that could occur here, some of them of which are related to timing, some of them of which are related to serendipity," Rep. Trent Franks, R-Ariz., said. "There is always a danger that it will go off the rails."

Mueller's appointment was widely praised by Democrats and Republicans.

But both sides of the aisle remember how past investigations led by special counsels ended up implicating individuals not assumed to be targets, or uncovering wrongdoing not assumed to be the in the scope of the probe when it first began.

President Bill Clinton's affair with Monica Lewinsky was discovered as part of an investigation into his and wife Hillary Clinton's finances. Scooter Libby, aide to Vice President Dick Cheney, was the only official prosecuted in the investigation into who uncovered the identity of covert CIA agent Valerie Plame, even though State Department official Richard Armitage was the leaker.

The Whitewater probe lasted four years; the Plame inquiry took two. That's why Republicans also fret about timing. The Russia investigation could finish just before the midterm elections, or in the run up to 2020, a major problem for the GOP if it doesn't fully exonerate Trump and his associates.

"The concern of people like us, is that this investigation will drag on and pop on us in October of 2018, and totally screw us," said a Republican strategist, who requested anonymity in order to speak candidly.

"I just don't like the idea that this could be strung out, strung out, strung out, strung out. I think it becomes a really big distraction," added Rep. Mike Kelly, R-Pa., a key Trump ally on Capitol Hill.

Republicans have been at the mercy of a chaotic, undisciplined White House for the past two weeks.

First, Trump's abrupt firing of James Comey and suggestions that he dismissed the FBI director because he was unhappy with the bureau's ongoing investigation into Russian meddling in the 2016 contest and potential collusion by the Trump campaign.

Then, came revelations that the president might have previously pressured Comey to drop the investigation into Michael Flynn, his former national security adviser. Later there was news that Trump shared classified intelligence with Russian officials during an Oval Office meeting.

Finally, there was a report that Trump told the Russians during that same meeting that firing Comey relieved the pressure he was facing on the Russia probe.

It's all House and Senate Republicans have been asked about, not to mention questions related to what they intend to do about it. That's why, once Deputy Attorney General Rod Rosenstein appointed Mueller, so many of them were legitimately enthusiastic.

To a degree, they still are. Now, Republicans hope, they can get attention for work they're doing to address voters' priorities like job creation, as opposed to every fresh Trump Twitter post.

"I'm focused more on working on the bread and butter issues," said Rep. Barbara Comstock of Virginia, a top Democratic target in 2018. "This will enable us to go back to our work."

Trump has adamantly denied doing anything improper or illegal, complaining that he's the victim of a political witch-hunt. The Mueller-investigation might determine as much.

Yet the issue has taken a political toll, especially Trump's habit of calling attention to it through provocative counterattacks.

The president's job approval numbers have been driven even lower, and now sit below 40 percent in the Real Clear Politics average. Even Rasmussen, which usually shows much higher ratings for Trump, showed him at 44 percent.

If the atmosphere doesn't change, congressional Republicans could have a real reason to worry about 2018. The first test of that is coming up in competitive special House elections in Montana (on Thursday) and Georgia (on June 20.)

As of Friday, the Democrats led the average of polls testing the generic congressional ballot by 7 percentage points.

Republicans remain skeptical that the appointment of a special counsel will sufficiently push the matter of Trump and Russia aside to insulate them from political fallout. That's partly because probes in House and Senate committees are continuing.

"We've got multiple committees and subcommittees trying to get a piece of this investigation," Senate Majority Whip John Cornyn of Texas said. "I'm sure there are going to be a lot of people wanting to talk about what the president just said, or some rabbit trail, but we've got to stay focused."