A little more than a year before the next round of congressional elections, at least 25 Senate incumbents and challengers have raised $1 million or more for their campaign war chests, second quarter reports now available at the Federal Election Commission show.

The filings are a testament to the power of incumbency: All but one of the 25 members of the million-dollar club are either incumbent senators or House members seeking a promotion to the upper chamber of Congress.

At the top of the heap are two early-bird candidates: Sen. Ed Markey, D-Mass., who just won a special election to fill the vacancy left when John Kerry left the Senate to become secretary of state. Coming in a close second Cory Booker, the celebrity Democratic mayor of Newark, N.J., who is running for in another special election this fall to replace the late Sen. Frank Lautenberg, D-N.J. Booker, who faces veteran Reps. Frank Pallone and Rush Holt in the Aug. 13 Democratic primary, has raised more than $6.5 million: Political Party Time shows he began fundraising well before he announced his candidacy June 8. Markey, despite raising more than $9.13 million still has some fundraising to do to meet his campaign's deficit of more than $2 million. 

Early campaign disclosures show a number of Senate Democrats preparing for tough races. They include: Kay Hagan of North Carolina, Mary Landrieu of Louisiana, Mark Begich of Alaska and Mark Pryor of Arkansas, whom the Cook Political Report put on the endangered list Wednesday, following word of an impending challenge from Rep. Tom Cotton, R-Ark., a Harvard Law School grad who interrupted his legal career after 9/11 to serve in the infantry in Iraq.

Another race in the spotlight is between Senate Minority Leader Mitch McConnell, R-Ky., and Kentucky Secretary of State Alison Lundergan Grimes, a candidate Democrats believe would be best suited to unseat the longtime senator by portraying him as one of the causes of the constant gridlock in the Senate. While McConnell’s fundraising so far totals $3.33 million, Grimes is yet to file her first report with the FEC, having filed candidacy papers only in mid-July.

Meanwhile, super PACs are ahead of the curve in the state. Pro-McConnell super PAC, Kentuckians for Strong Leadership recently released an ad against Grimes a day after her formal announcement that parodied a Grimes campaign ad featuring her grandmother. In the original ad, the grandmother says, "What rhymes with Mitch? It’s time to switch." One of the donors to the Kentucky super PAC is Super PAC for America, which gave $100,000 in June.  Some other donors include Donald Trump and the recently deceased Texas businessman and major political donor, Bob Perry. Here's the pro-McConnell PAC's version:

Two other left-leaning super PACs Senate Majority PAC and Patriot Majority PAC have entered the fray running ads against McConnell.

Today is the deadline for most super PACs to file campaign finance reports covering the first half of the year; Sunlight's Reporting Group will have a round up of the big players and donors tomorrow.

See below for Senate candidates with the most cash. Non-incumbents are in bold: