The words or phrases that best contribute to a romantic mood are: "pledge with mine" and "not change for thine."
A romantic mood is the feeling of love that is sparked in the reader due to the words that the author uses to communicate his feelings.
In this excerpt from the song, we see that the use of phrases such as "pledge with my eyes" and "not change for thine" communicate the speaker's unfailing love for the person whom he sings.
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