The GOP-controlled Congress returns Monday in what members and top staffers say will be one of the busiest Junes in years — as Republicans try to pass ObamaCare reform or another top item on President Trump’s legislative agenda.

Their goal to give Trump -- and themselves -- a major win during the president’s first year in office continues to be complicated by additional legislative challenges and the ongoing Capitol Hill investigations into whether the Trump campaign colluded with Russia during the 2016 presidential elections.

Lawmakers are way behind on the annual spending legislation to keep the government fully operational past September and likely will have to pass another stop-gap measure.

In addition, they recently were informed by Treasury Secretary Steven Mnuchin that they will have to raise the federal government's borrowing limit before August, a daunting task ripe for brinkmanship.

Senate Republicans say they are working daily behind closed doors to craft an ObamaCare overhaul bill, following the House last month passing its version. However, Republicans appear less than optimistic about crafting a bill that at least 51 of its 52 senators will sign.

“I don't see a comprehensive health care plan this year," North Carolina GOP Sen. Richard Burr, chairman of the Senate's Intelligence committee, on Friday told a hometown TV station. "At the end of the day, this is too important to get wrong."

Still, Trump and essentially every elected Washington Republican campaigned on repealing and replacing ObamaCare. So failing in that effort would be a big problem with voters, ahead of the 2018 midterm races in which Democrats are trying to win about two dozen more House seats to retake the chamber.

"We just need to work harder," Texas Sen. John Cornyn, the Senate’s No. 2 Republican, told KFYO radio in Lubbock over the week-long congressional recess that ends Sunday.

And he pledged to complete the health care “by the end of July at the latest."

Congress has yet to unveil a plan to overhaul the U.S. tax code -- another Trump campaign promise -- even though the president recently tweeted that the plan is ahead of schedule.

"The president keeps saying the tax bill is moving through Congress. It doesn't exist," House Minority Leader Nancy Pelosi, D-Calif., said mockingly on Friday.

Seven legislative weeks are left before Congress scatters for the five-week August recess.

Healthcare and taxes are enormously difficult challenges, and the tax legislation must follow -- for procedural reasons -- passage of a budget, no small task on its own.

Looming over everything is the investigation into allegations of Russian meddling in the 2016 campaign and connections with the Trump campaign.

Former FBI Director James Comey, who was fired by Trump, is scheduled to testify before the Senate on Thursday.

"The Russia investigation takes a lot of oxygen, it takes a lot of attention," said Republican Rep. Tom Cole of Oklahoma, a veteran lawmaker.

Trump has hired an outside attorney and reportedly dedicated an entire team to the issue -- in an apparent attempt to limit the amount of distraction the issue is creating for his legislative agenda.

Cole also argued that Republicans have not gotten the credit they deserve to date for what they have accomplished: voting to overturn a series of Obama regulations and reaching compromise last month on spending legislation for the remainder of the 2017 budget year that included a big increase for defense.

The biggest bright spot for the party and for Trump remains Senate confirmation in early April of Supreme Court Justice Neil Gorsuch, whose elevation goes far to placate conservatives frustrated with inaction on other fronts.

Historically, Capitol Hill has been at its busiest and most productive in the early days of a new president's administration, during the traditional honeymoon. But with his approval ratings hovering around 40 percent, Trump never got that grace period, and although his core supporters show no signs of abandoning him, he is not providing the focused leadership usually essential to helping pass major legislation.

In the Senate, Republicans' slim 52-48 majority gives them little room for error on healthcare and taxes, issues where they are using complicated procedural rules to move ahead with simple majorities and no Democratic support. Trump's apparent disengagement from the legislative process was evident this past week when he demanded on Twitter that the Senate "should switch to 51 votes, immediately, and get Healthcare and TAX CUTS approved, fast and easy."

In fact that's exactly how Republicans already are moving. But the trouble is within their own ranks as Senate Republicans disagree over how quickly to unwind the Medicaid expansion under Obama's health law as well as other elements of the GOP bill.

For some Republicans, their sights are set on the more immediate and necessary tasks of completing the annual spending bills that are needed to avert a government shutdown when the budget year ends September 30, and on raising the debt ceiling to avert a first-ever default.

The Associated Press contributed to this report.