By Steve Holland and Doina Chiacu

WASHINGTON (Reuters) - Republican Donald Trump's choice of Indiana Governor Mike Pence as his vice presidential running mate on Friday helped bolster support among some conservatives skeptical about his policies, a crucial step as he prepares to accept the party's nomination next week.

Some conservatives who had fought against Trump's ascendancy in the Republican nominating race welcomed his announcement that he had picked Pence, a well-known social and evangelical conservative.

The Club for Growth, a conservative economic group, had sharply criticized Trump's support for protectionist trade policies. On Friday, the group noted that as a member of the House of Representatives, Pence had been a strong voice for "free markets and economic liberty" at a time when the Republican leadership had been weak on these issues.

"Today’s news gives a similar hope that Mike Pence will be effective in pulling the Republican ticket toward economic conservatism and limited government," the group said.

Other conservatives were also heartened by Trump's VP pick, which the businessman announced in a tweet ahead of a joint appearance in his hometown of New York on Saturday.

"Pence is a principled conservative, man of faith, and talented messenger for Republican ideas," said Senate Majority Leader Mitch McConnell. "His addition to the ticket will bring even more excitement to the voters who are eager to put a Republican in the White House and deny a third term for President Obama's liberal agenda."

Trump, 70, chose Pence, 57, over two politicians he considers friends and close advisers, former House of Representatives Speaker Newt Gingrich, 73, and New Jersey Governor Chris Christie, 53.

Pence is viewed as a safe choice for Trump. The Indiana governor's low-key demeanor is a contrast to that of the bombastic real estate developer. And he could take the fight to the Democrats in the months leading up to the Nov. 8 election without upstaging Trump.

He and Trump will have to smooth over some policy differences. Pence has said he does not support trade protectionism or a temporary ban on Muslims entering the United States, two policies that are central to Trump's campaign message.

In a Fox News interview on Thursday night, Trump said Pence had done a great job in Indiana and that of all the people he had interviewed for the job, "there's nobody that agrees with me fully on everything."

Pence's selection was slammed by Democratic presidential candidate Hillary Clinton's campaign.

“By picking Mike Pence as his running mate, Donald Trump has doubled down on some of his most disturbing beliefs by choosing an incredibly divisive and unpopular running mate known for supporting discriminatory politics and failed economic policies that favor millionaires and corporations over working families,” said Clinton campaign Chairman John Podesta.

Trump had faced a midday Friday deadline to announce Pence because the governor had to declare by then whether he would be on the ballot in his home state for re-election.

Trump had postponed his announcement, which had been planned for Friday, out of respect for the victims in Nice, France. An attacker in a heavy truck drove into crowds celebrating Bastille Day, killing at least 84 people and injuring scores more in what President Francois Hollande called a terrorist act.

Trump is set to be formally nominated as the party's candidate for the presidential election at the convention, which opens in Cleveland on Monday. Traditionally, the vice presidential choice is used to build enthusiasm among party loyalists.

The Republican National Committee expects the convention to draw 50,000 people to the Ohio city, whose population is about 390,000. U.S. authorities were preparing for the possibility of violence, whether from demonstrators or planned attacks.

The Cleveland conclave and the Democratic Party Convention the following week in Philadelphia have been given the status of special national security event by the federal government for the first time, and security will be heightened, said Republican Party spokesman Sean Spicer.

The Department of Homeland Security will send more than 3,000 personnel to each convention, Secretary Jeh Johnson said on Thursday, speaking before the Nice attack. No specific or credible threat to either gathering has been reported, he said.

(Editing by Frances Kerry and Jonathan Oatis)