The word "rival" is correct and usable in written English.
It can be used to refer to two competing individuals or teams that are vying for the same prize or position. For example: "The two top teams were rivals in the championship match."
In reality, the effect on MPs' seats is more complex to measure as voters are not spread evenly across constituencies: some areas within may heavily favour one party, others its main rival.
Their talk was of rival software packages and camera types, and the scholarships to western universities they always just seemed to miss.
Johnson, seen as a political rival to Osborne, trumpeted the report to the skies.
The Guardian has unveiled its Australian digital edition with a pledge to deliver a "fresh and independent view", starting with an exclusive interview with prime minister Julia Gillard who criticises rival Tony Abbott's flagship paid parental leave policy as "anti-women" – and revealing that she is a secret Game of Thrones fan.
Just a 10-minute stroll from the Short Strand, hundreds of people looked on as two rival factions jeered and snarled at one another before the final confrontation, when hand-to-hand fighting finally broke out.
Yellen's studied, academic, consensus-friendly approach is also another plus in her favor – particularly in comparison to Larry Summers, her bright but brash rival for the job.
Rather disappointingly he chose to be the bigger man and merely cantered past his rival without so much as a sideways look.
INDIEGOGO iOS/Android Indiegogo is the main rival to crowdfunding website Kickstarter: a place where musicians, film-makers, gadget makers and others try to raise money for their grand schemes.
Tim Vine Milton Jones's only rival for the title of Britain's punner-in-chief takes to the road with a reprise of his 2014 Edinburgh fringe show, Tim Timinee Tim Timinee Tim Tim to You.
Far more tourists head for Skye and its famous Cuillin ridge just across the water, but there's plenty in Knoydart to rival it for adventure.
The underlying issue, therefore, is the rival claims of king and would-be caliph.
