CNN News:美国总统大选首战爱荷华州党团会议拉开大幕

We are exactly one week away from the Iowa Caucuses. They're the first formal contest in the U.S. presidential nomination process. Caucuses and primaries are state held events that help narrow down a field of candidates so that each major party has only one name that appears on the presidential ballot. For Republicans there are two people challenging incumbent President Donald Trump for the party's nomination. One additional candidate has dropped out of the race but election analysts expect the U.S. leader will easily win his party's contest.
For Democrats, it's more complicated. There are currently 12 of them still in the running and 16 other Democrats have dropped out of the race. For those still in it, all eyes are on Iowa. When voters there caucus on February 3rd, they could give a big lift to the candidate to make a strong showing.

According to the Des Moines Register, more than half of Democrats and Republicans who won the Iowa Caucuses have eventually won their party's nomination for president at least since 1972. But only three of those Iowa Caucus winner actually went on to become president. So why do the matter so much right now? Momentum.
Doing well in the first contest gives candidates a boost. It gets their names out there. It helps them with fundraising. And for those who don't do well in Iowa, the results can have the opposite effect.
They may struggle to gain ground on the winners. They may struggle to get campaign donations. Some candidates usually drop out after Iowa.