Michelle Nunn, Democratic candidate for Senate in Georgia, outraised her Republican competition in the past quarter. (Kevin Wolf/AP Images For A Billion + Change)

Donors gave big money to a trio of Democratic women running in battleground U.S. Senate races between January and March, a much-needed jolt of energy for Senate Democrats looking at an increasingly difficult path to holding their fragile majority in the midterm elections.

By the end of Tuesday’s deadline for congressional candidates to submit first-quarter fundraising totals, Sen. Kay Hagan (N.C.), Georgia’s Michelle Nunn and Kentucky Secretary of State Alison Lundergan Grimes were three of the biggest standouts. They each outraised their Republican competition and brought in totals exceeding $2 million.

Super PACs also filed impressive numbers, as did a pair of Republican challengers running in conservative states held by Democrats. Tea party Republicans hoping to dislodge GOP incumbents, meanwhile, struggled to keep pace.

The numbers came rolling in against the backdrop of an emerging television advertising war that is focusing on Democratic candidates. Americans for Prosperity, a group backed by the billionaire industrialist Koch brothers, has spent at least $30 million on congressional races so far. In North Carolina, AFP spent at least $7 million against Hagan through the end of March.

Kentucky and Georgia seem to be Democrats’ two best chances to pick up Senate seats on a map that otherwise has them on defense. The outcome in North Carolina promises to factor heavily into which party is in the majority come 2015.

Sen. Mary Landrieu's (D-La.) Senate campaign ad touting her advocacy of the oil and gas industry. (Landrieu for Senate via YouTube)

Democratic women running in those states are backed by Emily’s List, which supports pro-abortion-rights female candidates. Through bundled donations or contributions from its political action committee, Emily’s List has given $5.5 million to its endorsed candidates this election cycle.

In Kentucky, Grimes raised $2.7 million, besting Republican Senate Minority Leader Mitch McConnell. In Georgia, Nunn raised $2.4 million, lapping the field of Republicans there, several of whom put up lackluster numbers. Hagan raised $2.8 million, more than double the leading Republican candidate, state House Speaker Thom Tillis. In Louisiana, site of another key showdown, Democratic Sen. Mary Landrieu outpaced her leading Republican opponent, Rep. Bill Cassidy, by about $600,000.

Senate Republicans need to pick up six seats to win the majority. In two of their best chances to make gains — Alaska and Arkansas — Republican challengers outraised Democratic incumbents. Former Alaska attorney general Dan Sullivan raised $1.3 million, outpacing Sen. Mark Begich, who brought in just over $1 million. Rep. Tom Cotton edged out Sen. Mark Pryor by about $130,000 in Arkansas.

Tea party candidates running against Republican incumbents struggled to raise cash coming out of the gates this year. Club for Growth-backed lawyer Bryan Smith (R) raised $137,000 in the first quarter, only about a third of the money raised by Rep. Mike Simpson (R-Idaho), the incumbent he is trying to unseat. Mississippi state Sen. Chris McDaniel (R), who is backed by the Club for Growth and the Senate Conservatives Fund, pulled in less than a third of the total that Sen. Thad Cochran raised. Meanwhile, several of the Republicans running against Sen. Lindsey O. Graham in South Carolina finished well off his pace.

The first-quarter fundraising period marked a renaissance for super PACs, which looked to be taking a back seat to nonprofit groups such as AFP. Senate Majority PAC pulled in $11 million to help Democrats keep the Senate, and the leading GOP super PAC, American Crossroads, awoke from its slumber to raise more than $5 million in March alone. Both totals trumped those super PACs’ hauls from the entire 2013 calendar year.

Even as outside groups are playing an increasing role in campaigns, candidate fundraising still matters. Candidates have access to better advertising rates, and fundraising totals are typically a good gauge of candidate momentum.

Boosted by fundraising appearances by President Obama, the House and Senate Democratic campaign arms continued to raise big money. The Democratic Congressional Campaign Committee pulled in more than $10 million in March alone; the Democratic Senatorial Campaign Committee raised more than $8 million last month, besting its GOP counterpart by about $1.75 million.

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The National Republican Congressional Committee has not yet released its March fundraising totals. But Republicans are not at serious risk of losing their House majority. Democrats need to pick up 17 seats to wrest control of the lower chamber, a goal strategists in both parties acknowledge is all but impossible.

Aaron Blake contributed to this report.