When Judd Gregg comes to Capitol Hill as the new Commerce secretary, the conservative New Hampshire Senator and trusted GOP leadership adviser will undoubtedly be viewed with suspicion as he tries to sell a Democratic administration’s initiatives to his one-time Republican colleagues.

President Barack Obama’s decision to tap Gregg — a pragmatic fiscal hawk who has been a reliable GOP vote in the Senate — has been hailed as a brilliant political move. But for Gregg, entering a Democratic president’s Cabinet is likely to constrain him in ways he never experienced as a prominent and influential Senator.

“Obviously, he will no longer be a free agent. He will be carrying whatever water … the president gives him to carry,” said Sen. Bob Bennett (R-Utah), who, like Gregg, serves as an informal adviser to Senate Minority Leader Mitch McConnell (R-Ky.).

Bennett added, “Now I expect that he will be a voice of great reason and stability within the counsel to the administration, and I think the president will be well-served to listen to what he has to say. But in the end, [once] he’s a Cabinet officer, the decision as to what position he will take and what he will have to say will be the president’s decision, not his. That’s the way the system works.”

But there’s no question that Obama views Gregg as a potential ambassador to Congressional Republicans — someone who can communicate Obama’s policies and build across-the-aisle support.

“He will be a bipartisan bridge,” one White House official said.

Gregg, however, already seemed poised to break with some of his GOP colleagues when he praised Obama’s economic stimulus plan at the press conference formally announcing his nomination.

“You’ve outlined an extraordinarily bold and aggressive, effective and comprehensive plan for how we can get this country moving,” Gregg said, while standing alongside the president.

“This is not a time for partisanship. … This is a time to govern, and govern well,” the three-term Senator added. “And therefore, when the president asked me to join his administration … I believed it was my obligation to say ‘yes,’ and I look forward to it with enthusiasm.”

Most Senate Republicans have said they are leaning against voting for the massive $888 billion economic recovery package that is heavy on government spending on infrastructure and social services.

“I think he understands and respects the fact that we’re probably going to choose to disagree on … a good number of the initiatives the Obama administration puts forward,” Senate Republican Conference Vice Chairman John Thune (S.D.) said. “I suspect he contemplated all that before he decided to make the move.”

It was unclear when the Senate Commerce, Science and Transportation Committee would hold hearings on Gregg’s nomination. A spokesman said Chairman Jay Rockefeller (D-W.Va.) hopes to schedule something “soon.”

Many Republicans said the GOP Conference will always show deference toward Gregg, but they are skeptical that Obama truly intends to incorporate his ideas into major policy decisions. Considering that past presidents have alternately valued or ignored their Cabinet secretaries, many Republicans said they will wait and see how much power Obama allows Gregg.

“I guess it just depends on how much President Obama includes him in policy decisions,” said Sen. John Ensign (Nev.), the GOP policy chairman. “If the administration wants to reach out [to Republicans], this is one way to do it. … If it’s just window dressing, it won’t work. If this means that President Obama wants Republicans ideas — people around him that differ — then it could be very powerful for the Obama administration.”

Despite the substantial policy differences between Obama and Gregg, White House officials insisted Gregg would be an important economic adviser to the president, one who will get ample face time with the boss.

“Sen. Gregg will be a key member of the economic team,” the White House official said. “He will have access to the president.”

One senior House Republican leadership aide welcomed Gregg’s appointment but questioned how his conservative philosophy could be integrated into Obama’s agenda. The aide emphasized that Obama’s policies, and not Gregg’s presence in the administration, will ultimately determine whether Obama’s initiatives attract GOP support.

Some Senators indicated that they believe Gregg is more effective as a Senator than he would be as Commerce secretary in trying to convince Republicans to consider the administration’s proposals.

“He was already a strong force for the administration here, because he was willing to listen to what they had to say on the banking and credit issues,” Senate Republican Conference Chairman Lamar Alexander (Tenn.) said. “He was a force in forging a bipartisan compromise, and he did that from within the Senate. In some ways, he can’t do that from outside the Senate.”

Others said Gregg’s appointment holds the promise of creating a line for Senate Republicans into the administration, and around Senate Democrats.

“If the flow of information is two-way there, I think he could be useful in terms of helping to moderate the administration’s proposals, perhaps try and find some common ground and be useful in coming up with solutions where we could give bipartisan support,” Thune said. “I could see that happening. I don’t know. This is new ground for a lot of us.”