WASHINGTON — President Trump's strong-arm negotiating tactics may have worked against business opponents but they are backfiring with Republican senators, who resent being bullied to vote with the president on health care and other issues and have the political clout to resist him, experts say.

"No matter how strong or dominant a personality the president has, he is going to have trouble taking on an American political institution as powerful as the U.S. Senate," said Grant Reeher, a political science professor and director of the Campbell Public Affairs Institute at Syracuse University. "Senators have a strong sense of independence and sense of self that says 'I don't get pushed around that way.' And they're pushing back."

In the last two weeks alone:

• Sen. Lisa Murkowski, R-Alaska, chairman of the Energy and Natural Resources Committee, helped defeat Trump-supported bills to repeal Obamacare despite reported threats from the administration that her vote could jeopardize Alaska's economic future. The warnings were delivered by Interior Secretary Ryan Zinke, who must go before Murkowski's panel for budget and staffing approval.

• Judiciary Chairman Chuck Grassley, R-Iowa, made it clear that his committee will not approve a new attorney general for Trump if he fires Jeff Sessions. Senators have rallied around the former Alabama senator amid blistering attacks against him by the president, who is angry that Sessions recused himself from the investigation of possible collusion between the Trump campaign and Russian officials.

• Majority Leader Mitch McConnell, R-Ky., said the Senate will not get rid of the legislative filibuster, despite continued tweets from Trump pressuring GOP senators to eliminate the rule in order to make it easier for the White House to push through its agenda.

"It's stunning to think the president believes that this kind of pressure campaign is going to bring senators to the table when it is actually repelling them," said Joshua Huder, a senior fellow at the Government Affairs Institute at Georgetown University. He said the threats against Murkowski were especially "dumbfounding."

Trump's tactics reflect a fundamental misunderstanding of the separation of powers and the fact that the legislative branch is equal to the executive branch under the Constitution, said James Thurber, a political scientist at American University and author of the book Rivals for Power: Presidential-Congressional Relations.

Rather than inspiring loyalty from GOP senators, Trump's "ham-handed, amateurish and stupid" tactics may spur them to increasingly go their own way, Thurber said. Just last week, the Senate approved economic sanctions against Russia despite Trump's objections. Trump reluctantly signed the bill even though it restricts his power.

"I see the president being more and more marginalized, with the leadership in the House and Senate going forward with their own agenda, and trying to ignore him as best they can," Thurber said.

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Trump's plunging popularity is emboldening senators to exercise their independence, Thurber said. A poll released Wednesday by Quinnipiac University showed that only 33% of Americans approve of how the president is doing his job.

"He's not in a position to inspire fear or respect," Thurber said.

In contrast, Sen. Susan Collins, R-Maine, who consistently opposed Trump on Obamacare repeal bills that would have defunded Planned Parenthood or cut Medicaid for her constituents, was listed among the 10 most popular senators in America, according to the non-partisan Morning Consult's senator approval rankings for July.

In Trump's defense, he is not the first president to try to pressure senators to do his bidding, but he may be the most blatant, Reeher said.

"LBJ could also take a bullying approach, but he did it in the Oval Office, not publicly like Trump has been doing," the professor said, referring to former president Lyndon Johnson. "Nixon used to take members of Congress out on the presidential yacht (which no longer exists) and run them up and down the Potomac River with their spouses and schmooze them."

Trump might do well to reject both of those extremes and spend more time sitting down with individual senators and listening to their concerns, as he has sometimes done in private meetings at the White House, Thurber said.

"“Stop tweeting, work quietly with them, build coalitions," Thurber advised. "He needs to support them as much as he can even though sometimes he doesn't totally agree with them."

It's not clear whether Trump's ego will allow him to take a gentler approach, said Eric Herzik, chairman of the political science department at the University of Nevada, Reno.

"I'm not sure Trump is ever going to get it," Herzik said. "He thinks he is truly in charge and the Senate should just do what he says."

Sen. John McCain, R-Ariz., made an impassioned speech to his colleagues last week about why the Senate shouldn't take orders from the White House, no matter who is president. He spoke three days before casting the deciding vote against the repeal of the Affordable Care Act. Trump lobbied McCain by phone to vote "yes" just minutes before McCain voted "no."

"Whether or not we are of the same party, we are not the president's subordinates, we are his equal," McCain declared. "The success of the Senate is important to the continued success of America. This country — this big, boisterous, brawling, intemperate, restless, striving, daring, beautiful, bountiful, brave, good and magnificent country — needs us to help it thrive."