WASHINGTON — Senator Mitch McConnell is usually impervious to criticism, even celebrating the nasty nicknames critics bestow on him. But Mr. McConnell, the Senate majority leader, is incensed by the name “Moscow Mitch,” and even more miffed that he has been called a “Russian asset” by critics who accuse him of single-handedly blocking stronger election security measures after Russia’s interference in 2016.

Democrats had been making the case for months, but it was supercharged last week by the testimony of Robert S. Mueller III, the former special counsel, who told the House Intelligence Committee that the Russians were back at it “as we sit here.”

Mr. McConnell cites several reasons for his opposition — a longstanding resistance to federal control over state elections, newly enacted security improvements that were shown to have worked in the 2018 voting and his suspicion that Democrats are trying to gain partisan advantage with a host of proposals.

Republican colleagues say that Mr. McConnell, a longtime foe of tougher campaign finance restrictions and disclosure requirements, is leery of even entering into legislative negotiation that could touch on fund-raising and campaign spending.