BRASILIA, Brazil (Reuters) -- Brazil's largest political party voted on Thursday to join the coalition government of President Luiz Inacio Lula da Silva in his second term, giving the former labor leader vital support for his legislative agenda.

The centrist Brazilian Democratic Movement Party (PMDB), which only erratically backed Lula in his first term, said it agreed with a seven-point agenda proposed by Lula and would officially form part of the government after Jan. 2.

"We are a large and eclectic party but we decided to unite and participate in the decision-making of the executive," Renan Calheiros, PMDB leader and Senate president, told Reuters after the vote.

In Oct. 1 elections, the PMDB won the most seats Congress and will give Lula roughly 50 votes beyond an absolute majority in the Lower House. But the former factory worker, who won re-election by a wide margin in an Oct. 29 run-off, would still fall short of the three-fifths majority needed for constitutional changes.

And a half dozen dissident PMDB senators may make passing the agenda more difficult in the Senate.

The Lula-proposed agenda backed by the PMDB includes tightening party discipline and campaign-finance rules to help combat corruption, restricting government expenditure growth and targeting 5 percent economic growth annually.

The core of Lula's next coalition government is made up of five parties, including his own Workers' Party, the PMDB and three smaller left-wing parties. Three other mid-sized centrist and center-right parties are expected to support government initiatives but not hold cabinet posts.

Despite public displays on Thursday of new-found unity in the PMDB, divisions between opposition and government supporters within the party remain and could undermine Lula's legislative agenda in the Senate.

A group of six PMDB senators has publicly opposed the alliance with Lula as a result of a series of corruption scandals involving the president's ruling Workers' Party (PT).

"Lula made a series of ethical mistakes and he ought to prove he learned from them before we pledge our allegiance," PMDB Sen. Geraldo Mesquita told Reuters. "We will maintain our independent vote," he added.

Without the six dissident PMDB votes, Lula would fall short of an absolute majority in the Senate with only around 37 of 81 seats.

Still, the PMDB deal could create a positive momentum for Lula in his efforts to win over a handful of senators from smaller opposition parties, some analysts say. "The alliance with the PMDB is a show of force," said Carlos Lopes, a Brasilia-based political analyst. "The numbers in Congress may not add up yet for Lula but you cannot underestimate the support of the country's largest party."