Democratic outside groups, which largely held their fire in the fall, have begun to increase their spending on behalf of Senate candidates while the party moves aggressively to tie Republican candidates to wealthy donors like the Kochs. According to media buyers, these Democratic groups spent about $4 million in March, led by Senate Majority PAC, a “super PAC” with ties to the Democratic leader, Senator Harry Reid of Nevada. Republican groups spent about $5.5 million.

The Democratic counterattack is being led chiefly by super PACs, the groups legalized after the Supreme Court’s Citizens United decision to allow big donors, unions and corporations to raise and spend unlimited contributions. By contrast, Republican super PACs, which have dominated fund-raising in the past two elections, appear to be collecting and spending less this time.

Instead, Republicans are relying heavily on nonprofit groups that do not disclose their donors and whose political activities have come under scrutiny by the Internal Revenue Service and congressional Democrats.

Several Republican challengers will need to spend more heavily in the weeks ahead to beat back rivals for the party’s nomination, or to introduce themselves to voters against Democrats who are already well known.

In the high-profile Senate race in Kentucky, neither Senator Mitch McConnell, the Republican leader, nor Alison Lundergan Grimes, his Democratic challenger, has released recent figures. But over the last six months, Ms. Grimes has steadily narrowed her financial gap with Mr. McConnell, who faces a primary challenge and attacks from conservative groups that are seeking to unseat him. In Republican-leaning Georgia, which has an open Senate seat, the Democratic candidate, Michelle Nunn, posted consistently strong fund-raising numbers during the second half of last year.

And some Republican candidates are floundering in the hunt for cash.

Senator Kay Hagan of North Carolina, a Democrat who has been the subject of nearly ceaseless attack ads from conservative groups since the fall, conserved about half of the cash she raised and began the spring with $8.3 million in hand. Thom Tillis, the Republican candidate backed by the party establishment, spent significantly more than he raised during the first three months of the year. Mr. Tillis, the speaker of the North Carolina House, began April with just $1 million in cash on hand and is locked in a primary fight.

In many general election matchups, spending by Republican super PACs and political nonprofit groups appears to have more than evened the financial playing field with Democrats.