McCaskill's office would not give any details about the attempted attack or say how they learned about it. A spokesman for Senate Majority Leader Mitch McConnell said he did not have an immediate comment.
Congressional Republicans who have put up with almost a year's worth of speculation and investigations into Trump's ties to Russia are hoping to move on from that topic in 2018, renewing focus on Democrat Hillary Clinton and stepping up criticism of Mueller.
The Massachusetts senator is the most prominent Democrat to have announced a run against President Trump in 2020. The two already have a long history of trading barbs, and it’s likely to get even nastier.