Swings...were

Straits Times - 16 January 2013

The highlighted verb is wrong. The correct verb is were.

As has been noted elsewhere in this blog, this kind of subject-verb agreement mistake is common because the head noun is obscured by intervening elements.

In the above, the head noun swings is buried in a noun phrase (NP) modified by a long prepositional phrase (PP):

[the swings [in voter sentiment among the undecided middle ground]PP]NP

The prepositional phrase contains two singular nouns (sentiment and ground), either of which could have been mistaken as the subject, leading to the wrong use of was.

Here's another one from the online ChannelNewsAsia:

ChannelNewsAsia - 23 January 2013

The correct verb is was since the head noun nature is singular.