WARSAW (Reuters) - Polish opposition parties have joined forces to try to win a majority in the upper house of parliament, the Senate, in parliamentary elections on Oct. 13, as they struggle to oust the ruling nationalist Law and Justice party (PiS) from power.

FILE PHOTO: People attend the Polish regional elections, at a polling station in Warsaw, Poland, October 21, 2018. REUTERS/Kacper Pempel

Opinion polls show PiS winning a second four-year term with more than 40 percent of the vote for the more powerful lower house, the Sejm, which is elected on a system of proportional representation based on party lists.

But the Senate is chosen on a system of first-past-the-post, whereby the candidate who wins most votes in a given constituency is duly elected. By agreeing not to put up rival candidates, the opposition parties increase their chances of defeating PiS.

“We believe that a list of jointly agreed candidates for the Senate offers us an opportunity to win the Senate elections,” Krzysztof Gawkowski, secretary-general of the leftist Wiosna party, told private Radio Zet on Sunday.

The Senate scrutinizes, debates and can reject legislation passed by the Sejm.

Critics say PiS has used its current majority in both the Sejm and the Senate to rush through bills without proper oversight or time to analyze their impact.

PiS, a socially conservative, euroskeptic party but which leans to the left on economic policy, hopes to win a two thirds majority in the Sejm in the October election, which would allow it to change Poland’s constitution.

But the opposition could block such a move if it held a majority of the Senate’s 100 seats.

The opposition groupings involved in the Senate deal include the centrist Civic Coalition and several leftist parties.

“If the opposition parties don’t compete with each other (in the Senate race) and unite behind one candidate in each district, they have a chance to win a Senate majority,” said Anna Materska-Sosnowska, a political scientist at Warsaw University.

PiS was quick to dismiss the opposition initiative. One PiS senator, Jan Maria Jackowski, told the Wyborcza.pl portal that according to his party’s analysis, PiS was still on track to win more than half of the seats in the Senate.

“On that side there are only negative emotions, their only program is that they are anti-PiS. We are a party that has a real program,” Joachim Brudzinski, an MEP and the head of PiS’ campaign team, told reporters on Monday.