I did some google research, and searched through the US Senate's website and found the following substantiating The Economist's claim about republican senator Orrin Hatch.

Proposing an amendment to the Constitution of the United States to make eligible for the Office of President a person who has been a United States citizen for 20 years. (Introduced in Senate)

The bill was introduced by Senator Hatch on July 10, 2003. Arnold Schwarznegger became a US citizen in 1983, so if this amendment passes he will be eligible to be elected as president of the country.