Rep. Shelley Berkley, the Democratic nominee for the Nevada Senate seat once held by scandal-plagued Republican John Ensign, is now embroiled in an ethics investigation of her own — making a top pick-up opportunity for her party a steeper climb.

Berkley is running to unseat appointed Sen. Dean Heller (R) in a presidential battleground state that is crucial to Democrats’ chances at holding both the White House and Senate. Her bid was clouded Monday by the House Ethics Committee announcement — less than four months from Election Day — that it is investigating whether Berkley broke any ethics rules or laws by saving a kidney transplant program at a hospital where her physician husband had a lucrative contract.

The Berkley campaign awoke this morning to front-page stories in both the Las Vegas Review-Journal and Reno Gazette-Journal — which cover the two vital population centers in the state — detailing the investigation. And she has already been forced to weather a spot from American Crossroads, a GOP-aligned super PAC, which ran an ad last month highlighting the ethics investigation. More ads are sure to follow.

The Berkley campaign had already been working to blunt the impact of the investigation, even before it was made official on Monday. The campaign launched a statewide TV ad on June 15 that attempted to tie Heller to the situation. “Shelley Berkley worked with Dean Heller, standing up to Washington bureaucrats who wanted to close Nevada’s only kidney transplant center,” the ad’s announcer says.

There hasn’t been much public polling on the race, but a NBC News/Marist poll conducted in late May found Heller and Berkley in a statistical tie.

The Democratic Senatorial Campaign Committee released a statement Monday indicating that the national party is not throwing in the towel. “Shelley Berkley is a proven fighter for her constituents and a proven winner on the campaign trail, and our support for her campaign remains as strong as ever,” DSCC Executive Director Guy Cecil said.

But national Republicans clearly feel the wind at their backs now and see a far better opportunity to hold one of the party’s three most vulnerable seats.

“Nevadans deserve someone in the Senate who they can trust to work on their behalf and not someone — like Ms. Berkley — who puts her own financial and political interests first,” said Rob Jesmer, executive director of the National Republican Senatorial Committee.

Here are the ads that began running last month: