Another Democrat has voiced concerns about a draft executive order that would require government contractors to disclose their political contributions.

Sen. Claire McCaskill (D-Mo.) — along with Sens. Joe Lieberman (I-Conn.), Susan Collins (R-Maine) and Rob Portman (R-Ohio) — signed a letter Thursday to President Obama expressing reservations about the draft order.

Lieberman is chairman, and Collins is ranking member, of the Senate Homeland Security Committee. McCaskill leads the panel’s Contracting Oversight subcommittee; Portman is the ranking member.

“We are concerned that requiring businesses to disclose their political activity when making an offer risks injecting politics into the contracting process,” the letter says. “The requirement that businesses disclose political expenditures as part of the offer process creates the appearance that this type of information could become a factor in the award of federal contracts.”

McCaskill joins other Democrats, including House Minority Whip Steny Hoyer (D-Md.), who have said they have problems with the draft order. Hoyer told reporters this week he was “not in agreement with the administration” on the draft order, saying it could politicize the bidding process and undermine its effectiveness.

The draft order comes after a flood of campaign advertising in the last election from outside groups, many of which did not have to disclose their donors. That followed the Supreme Court’s Citizens United decision in January 2010 that allowed corporations and unions to spend unlimited funds on political activities.

To counter that ruling, the White House and Democrats tried to move legislation that would have outside groups disclose their donors — called the DISCLOSE Act — but that stalled in the Senate.

Like the DISCLOSE Act, business groups and Republicans oppose the draft order, while several public interest groups support it.