Clinton's signature appears on a Tuesday email fundraising pitch that criticizes Republicans for catering to tea party conservatives and guarding corporate tax loopholes.
The Democratic two-term president calls fellow Democrat Marshall his friend and urges supporters to donate to an ad campaign.
Marshall is running against former state GOP chairman and former state legislator Mark Amodei in the special election set for Sept. 13.
The winner will replace Republican Dean Heller, who recently resigned from the House after he was appointed to fill the seat of U.S. Sen. John Ensign. Ensign resigned amid an ethics scandal involving an affair with a former staffer.