By Dave Graham

MEXICO CITY (Reuters) -Mexican President Andres Manuel Lopez Obrador this week intervened in the increasingly fractious race to succeed him, a move party insiders said was aimed at warding off potential division and protecting the commanding political power base he has built.

Under pressure for months from Foreign Minister Marcelo Ebrard to get contenders seeking the presidential nomination of the ruling National Regeneration Movement (MORENA) to step down before campaigning, Lopez Obrador this week finally went along with the idea, according to three party sources familiar with the matter.

In a dinner with party leaders in Mexico City on Monday, Lopez Obrador proposed that MORENA's contenders resign to ensure a level playing field, and that the party this weekend decide on rules for the selection process, the sources said.

"He's closing every loophole to prevent any disagreement," one of the sources said, speaking on condition of anonymity.

The president's office did not reply to a request for comment.

On Thursday morning, Lopez Obrador was asked about what was discussed at the MORENA dinner, and said: "We spoke of the need to remain united to guarantee the transformation of Mexico."

Ebrard, a leading contender to succeed Lopez Obrador, on Tuesday announced that he would step down next week. Ebrard's main rivals are likely to follow suit soon, the president himself suggested on Wednesday.

MORENA is expected to pick a candidate as soon as September. Mexico's next presidential election is in June 2024.

In public, the MORENA hopefuls have largely maintained a veneer of civility. But sniping between the rival camps behind the scenes has begun to erode the image of all-conquering unity the president tries to project for his party.

Under Lopez Obrador, MORENA in less than five years has replaced the Institutional Revolutionary Party (PRI) as the establishment party. On Sunday, it captured Mexico's most populous state after nearly a century of PRI rule.

However, in another election in the northern state of Cohauila that day, MORENA suffered a crushing defeat after infighting split its vote.

Lopez Obrador has been urging MORENA to fight for a two-thirds congressional super-majority next year.

That could allow him to push through contentious constitutional changes to the judiciary, which has persistently impeded his efforts to increase state control over the economy, before he leaves office on Sept. 30, 2024.

If MORENA is roiled by internal dissent, the prospect of such control looks remote.

UNITY

MORENA now controls the federal Congress, over two-thirds of Mexico's states, and opinion polls make it the hot favorite to win the 2024 presidential election.

Lopez Obrador's personal popularity has bolstered support for his party. But under Mexican law, presidents may only serve a single six-year term.

Because of MORENA's dominance, the contest to secure the party's candidacy has become a virtual de facto presidential election in the eyes of many analysts.

Most recent polling gives Mexico City Mayor Claudia Sheinbaum a slight edge over Ebrard in the succession battle, and senior aides to the president have told Reuters they believe she is Lopez Obrador's preferred candidate.

He has repeatedly denied this, and insisted that the process be transparent and beyond reproach.

Ebrard, a veteran aide of Lopez Obrador who succeeded him as Mexico City mayor in 2006, has aired concerns that rivals such as Sheinbaum with domestic political mandates have an advantage, and that all contenders should therefore give up their office.

Those concerns have fed the impression that Ebrard might leave MORENA for another party, which the president is eager to avoid, said Andres Rozental, a former deputy foreign minister.

Ebrard has dismissed talk he could leave MORENA.

"The most important thing for Lopez Obrador right now is to keep his movement united and alive," Rozental said.

As recently as Monday, Sheinbaum was saying she had no intention of stepping down before the MORENA selection process had concluded. She was unaware what the president would propose that very evening, according to one of the party sources.

Her office had no immediate comment.

Having to resign would take away Sheinbaum's main platform, working to Ebrard's advantage, Rozental said.

But he still expected her to prevail in the end - and for the president's exhortations for the contenders to play by MORENA's rules, and remain civil, to be obeyed.

Not to do so, he said, would be "almost suicidal."

(Reporting by Dave Graham; Editing by Mark Porter and Marguerita Choy)